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Hi people, I'm new using forums for this kind of purposes, so please forgive me if I can't explain myself well, also if this is the wrong sub-forum please tell me ¡.

I have to measure the output current & voltage of a High Frecuency, High Voltage transformer, I can measure the Output Voltage using a High Voltage Probe and my Handheld Oscilloscope, but I still have to deal with the current, I want to use a Homemade Shunt Resistor (0.1ohms) and my Oscilloscope, but before doing it I tried by measuring that voltage with my Fluke Multimeter, I was increasing the input voltage & It was working well but at some point my Multimeter Screen started to fade & show random numbers, What happened? The voltage on the Shunt wasn't that high (around 2.5 volts) when it started to fail, it still works. I don't want to damage my Oscilloscope, so I need to now what causes that issue.

Here is my circuit diagram if you wan't to know the way I'm measuring.

Considerations:
-I know my multimeter doesn't measure at those frequencies, but the error is constant so I can just adjust it.
-I know my oscilloscope does work at those frequencies.
-I have no idea of the value of the output current & voltage, also by measuring another transformer the values are not consistent.
 

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Wilko

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Hi - I’m back with the frequency being out of the meter’s range. Not sure what the point of measuring something that you know will be wrong?
 

marconi

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One needs to know more about V1 eg: waveform?, output impedance as a function of frequency - what is its Thevenin 'equivalent circuit'?

Similarly, one needs to know more about the transformer - what is its 'equivalent circuit'?

The Fluke multimeter I think is likely to be an unsuitable instrument for your measurements.

The secondary current you measure depends on the source of emf 'looking left into the secondary and its waveform', the impedance looking left into the secondary and the impedance of the load - plus the impedances and frequency & waveform characteristics of your instruments.

Need some more detail from you.
 
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Your measurement setup with the oscilloscope looks OK but safety first, the resistors suggest you have a high-voltage, high-current supply and if you're not secure with measurement principles I wonder if you have the expertise to be safe around the supply.

What is the output current and voltage and how are you protected from it?
 

Lucien Nunes

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Hmm, meter display going blank sounds a bit spooky and not obviously related to the measurement. Probably just coincidence.
 
  • Thread Starter Thread Starter
  • #7
Hi - I’m back with the frequency being out of the meter’s range. Not sure what the point of measuring something that you know will be wrong?
Hi, sorry it took me ages to reply but I'm kinda busy at work.
You're right, but I wanted to know if the error was somehow constant, & it was (kinda), I don't have access to any other meter so I'm stuck with it.
Here's the relation between the real output & what the meter shows.
 

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  • Thread Starter Thread Starter
  • #8
One needs to know more about V1 eg: waveform?, output impedance as a function of frequency - what is its Thevenin 'equivalent circuit'?

Similarly, one needs to know more about the transformer - what is its 'equivalent circuit'?

The Fluke multimeter I think is likely to be an unsuitable instrument for your measurements.

The secondary current you measure depends on the source of emf 'looking left into the secondary and its waveform', the impedance looking left into the secondary and the impedance of the load - plus the impedances and frequency & waveform characteristics of your instruments.

Need some more detail from you.
Hi, I know that should provide that kind of data, but sadly I can't show them, what I can show is this:
The input's waveform looks like a square (I'm attaching a pic & yes, where I'm working I only have access to Paint :l).
I still need to do the Thevenin Analysis, It's not that hard but they want me to focus on something else.
My instruments.... I'm stuck with them, I already measured the error, the results are shown in a reply above.
My output waveform looks like a M, I'm also attaching a pic showing it.
Thank you very much, you seem to know a lot.
 

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  • Thread Starter Thread Starter
  • #9
Your measurement setup with the oscilloscope looks OK but safety first, the resistors suggest you have a high-voltage, high-current supply and if you're not secure with measurement principles I wonder if you have the expertise to be safe around the supply.

What is the output current and voltage and how are you protected from it?
Hi, & thank you for replying <3.
I haven't had a proper training on that (if there's any), I have no idea about the output voltage & current (actually that's the reason I'm trying to measure it), but the voltage is around the 3.8KV.
About security.... there's none, we don't have security equipment.

Thank you very much.
 
  • Thread Starter Thread Starter
  • #11
Hmm, meter display going blank sounds a bit spooky and not obviously related to the measurement. Probably just coincidence.
Hi, & thanks for replying.
I know, I just wanted to be sure, I'll repeat the test again but not sure when, I have some others tasks to do, I'll post the results here if something happens.
 
Unfortunately yes, around £400.00 , is this a university project
 

marconi

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Re #8. The two waveforms tell you a lot. The equal mark:space ratio of the square wave means it contains only the fundamental sinusoid (20kHz) and odd harmonics of it ie: 60kHz, 100kHz, 140kHz,....

50231

The waveform of the output voltage/current shows it is mainly comprised of the fundamental (20kHz) and third harmonic (60kHz).

50232

Is your measurement question:

a. 'what is the effective root mean square(rms) value of this second waveform?' or

b. 'what is the amplitude of each component (fundamental, third, fifth, etcetera)? of this second waveform?'
 
  • Thread Starter Thread Starter
  • #15
Unfortunately yes, around £400.00 , is this a university project
Dang it, I can't afford it, kinda, it's more like a private project.
Thank you for the info btw.
 
  • Thread Starter Thread Starter
  • #16
Re #8. The two waveforms tell you a lot. The equal mark:space ratio of the square wave means it contains only the fundamental sinusoid (20kHz) and odd harmonics of it ie: 60kHz, 100kHz, 140kHz,....

View attachment 50231

The waveform of the output voltage/current shows it is mainly comprised of the fundamental (20kHz) and third harmonic (60kHz).

View attachment 50232

Is your measurement question:

a. 'what is the effective root mean square(rms) value of this second waveform?' or

b. 'what is the amplitude of each component (fundamental, third, fifth, etcetera)? of this second waveform?'
I knew that would happen, because the transformer acts (kinda) like a filter.
I'm aware of how VRMS works, I know that I could get it's function & evaluate it to obtain such voltage (& then check if that matches with the value shown in my oscilloscope), also, knowing the values of those armonics is not really important to me.

My main issue is if there are risks if I measure the current using a shunt & my oscilloscope, because I did a measure using such shunt & my multimeter & at some point it started to show some random numbers & the display faded, I need to measure the waveform using the oscilloscope & I don't want to damage it.

Btw, no, it doesn't have a FFT function (& you don't know how much I hate that).
 

marconi

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I think I better understand(?) your measurement problem now; how do you measure/detect a small ac current flowing in a high voltage circuit using an oscilloscope without risk of high voltage damage to the oscilloscope.

What one might do is provide galvanic isolation between the oscilloscope and the sensed circuit. A transformer does that as suggested earlier. You could also use a Hall effect current sensor. The problem I think you will find is discovering such a sensor which will measure small currents and provide sufficient galvanic isolation.

I once used this chip to measure ac current in a 240V circuit - it has 2.4kV of galvanic isolation but operates in the 0-10Amp range which is no use to you.


What I suggest you do is do some research and also contact manufacturers of such Hall Effect Sensors, Allegro being one.

Sorry I cannot be more help - but it is certainly an interesting problem - to measure a tiny current, in a high voltage circuit, accurately and above all safely. Is this why you have been set the problem?

You might want to look at this:


and think about how you could use an opto-isolator/opto-coupler between the current sensor and your 'scope:



A quick look at the isolation voltage shows some of the opto-isos in the kV range.

Good luck.

:)
 

DPG

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What is the reason for your requirement? What are you actually wanting to find out?
 
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